1. Field of the Invention
The present invention relates to a charge pump and, more particularly, to a charge pump circuit with bipolar output that can be applied to existent CMOS IC fabrication processes.
2. Description of Related Art
With the development of the manufacturing process, the size and operating voltage of components become smaller. However, the transmission voltages of I/O signals usually are higher than those of internal circuits or applied voltages. Therefore, it is necessary to design a DC voltage conversion circuit in an IC to provide a voltage source with a voltage higher than the applied voltage. Charge pump circuit is one of the DC voltage conversion circuit.
Because the charge pump circuits proposed here have the function of converting a unipolar voltage (+V) to a bipolar voltage output (+/−V) or a bipolar double voltage output (+/−2V), they can be widely used in ICs, e.g., RS-232 ICs. U.S. Pat. No. 5,306,954 proposed by Sipex Corporation, USA discloses a charge pump circuit with symmetric positive/negative voltage output capability, which is composed of two transfer capacitors, two storage capacitors, and nine switches. The operation of these switches adopts clock signals generated by means of oscillation triggering to drive four-phase switching. Moreover, U.S. Pat. No. 4,999,761 proposed by Maxim Integrated Products, USA discloses an integrated bipolar charge pump power supply and an RS-232 transmitter/receiver, in which a charge pump circuit is composed of two transfer capacitors, two storage capacitors, and eight switches. These switches are driven by two-phase clock signals.
Regardless of what type of charge pump circuits mentioned above, they have the drawbacks of both limited charge conversion efficiency and large ripple of output voltage. In particular, the four-phase switched charge pump circuit proposed by Sipex Corporation, USA has a larger ripple. Therefore, the manufacturers have been giving every effort to design a circuit structure of small size and high efficiency.
Accordingly, the present invention aims to propose a new charge pump circuit structure with bipolar output in order to solve the above problems in the prior art and create a high-efficiency circuit.